That's fantastic. Problem solved, thank you very much.
For anyone who finds this thread whilst trying to contact Asana, the following works:
req = HTTPRequest[
"https://app.asana.com/api/1.0/users/me", <|Method -> "Get",
"Headers" -> {"Authorization" -> "Bearer 0/abc123"}|>]
URLRead{req]
Please note, Asana requires a Get rather than a Post, hence the change to Neil's code. Also, using the Personal Access Token (PAT) means that you don't need to send the username and password